Dial Plans

The dial-plan object in Lync is responsible for normalization of numbers, which is the process of taking a set of numbers that a user enters and converting the numbers to some other format. The dial plan does not control what numbers a user can call or dictate where a call is sent. Each user enabled for Enterprise Voice is assigned a dial plan, which contains an ordered list of normalization rules that are processed from top to bottom looking for a matching rule.

The first step in placing a Lync call is to normalize the digits a user has entered to the E.164 format. Since regions and countries all dial numbers differently, this ensures that all numbers are in an equivalent format before any decisions on how the call is to be routed are attempted. Lync dial plans must contain rules to normalize the different types of calls users will make. For example, users in the United States are used to dialing seven-digit numbers for local calls from legacy systems, so the dial plan needs logic to convert the seven digits a user enters to E.164 by prepending the + sign, the North America region code, and an appropriate three-digit area code.

Each normalization rule in a dial plan consists of a pattern to match, and a translation pattern. The pattern to match is a regular expression pattern that searches against the number a user entered. If the dialed number matches the pattern, the rule is selected and the corresponding translation pattern applied. The translation pattern uses some part or all of the dialed number and manipulates it to the point where it matches the E.164 format. In the previous example the pattern to match would look for seven dialed digits, and the translation pattern would add the +1 and a three-digit area code. The originally dialed seven digits would be placed at the end of the string.

United States users also dial 10 digits for national calls, but can optionally dial 11 digits when including the North America region code, so the dial plan needs to allow for both of these options. Dialing an international number in North America is done by entering the digits 011 followed by the country code and the remainder of the number. In this case a normalization rule needs to add the + sign and then remove the 011 prefix in order to convert the number to the E.164 format. These examples are specific to the United States, but each country or region has its own standards or habits that normalization rules should accommodate. The end result is that any number a user dials should be converted to E.164 through the dial plan.

Organizations migrating to Lync Enterprise Voice will also need to configure rules for PBX extensions within the Lync dial plan. For example, many businesses use some shortened form of internal dialing such as three- to six-digit extensions that identify each user. This cuts down on the number of digits users need to dial when calling each other, and even allows users to memorize the extensions for each other.

This form of shortened dialing can be accommodated in Lync through the use of normalization rules within the dial plans. Just as with local, national, or international calls, the rules should search for a matching pattern and apply a translation pattern. For example, one rule might search for four digits and then apply a translation to prepend the + sign, a 1, a three-digit area code, and the three-digit local exchange. In this case a four-digit number entered by a user has been converted to a full E.164-formatted number. It’s very common for different internal extension ranges to correspond to different direct inward dial ranges as well, so the pattern 5xxx might need to normalize differently from the pattern 6xxx. Two separate normalization rules are required to handle the differing translation patterns for those scenarios. The number of normalization rules required in each dial plan will be driven by the number of DID ranges a company owns.

The preceding section described how to configure normalization for DID numbers, but some organizations do not allow DID calls, and instead use extensions that are not reachable directly from the PSTN. Outside callers typically route through a single main listed number, which goes to a receptionist or an automated attendant that can transfer callers to the internal user. Lync can accommodate these scenarios as well, by use of a format within the translation rule that specifies an extension. The translation pattern prepends an E.164-formatted main-line number, followed by the string “;ext=” and the user’s extension. For example, assuming that the main line is +12345678900, the normalization rule would convert the extension 5555 to +12345678900;ext=5555. The Lync client displays the number in a much cleaner format to the users, so to them the dialed number appears as +1 (234) 567-8900 (X5555).